Modern telecommunication networks make use of a set of signalling protocols known collectively as SS7 to set up and control calls. SS7 messages are exchanged between so-called Signalling End Points which may be, for example, local exchanges of a Public Switched Telephone Network (PSTN), Mobile Switching Centres (MSCs) of a GSM network, or Radio Network Controllers of a UMTS (3G) Radio Access Network. SEPs are identified by unique Signalling Point Codes (PC) which act as addresses within the SS7 network. Other network nodes may also be allocated unique PCs to allow SS7 signals to be routed to these nodes.
Setting up or changing a traditional SS7 based signalling network is a laborious task involving the dimensioning and defining of links, link-sets, routes, route-sets, routing data, etc. This needs to be done in every node, which is part of the network. To avoid a full-mesh, which would be very expensive from both an administrative and transmission network point of view, traditional SS7 signalling networks have been built according to a hierarchical network structure. This structure is illustrated in FIG. 1. Every Signalling End Point (SEP) has usually been connected to a pair of Signalling Transfer Points (STPs). This solution has allowed the SEPs to have only limited knowledge about other SEPs and their location in the network. The STPs have been the points in the network where the network structure has been known, which has allowed the STPs to do the routing of signalling messages between SEPs.
Within an SS7 network routing can be based on a destination PC (and optionally a subsystem number (SSN)). When a destination PC/SSN is provided for a signal, each participating node (e.g., switches, STPs, SCPs, etc.) within the serving network must have data identifying the specified destination PC/SSN. Therefore, whenever a signal is received with a particular destination PC/SSN, each transferring node within the serving network knows exactly where to send the signal.
Signals can also be routed using global title (GT) numbers. When the node originating a signal does not know the destination PC associated with a particular destination node, a global title number has to be used for routing purposes. That global title number is translated into an “intermediate” PC which identifies the next known step. Translation is a two step process involving a first translation from the global title number to a global title routing case (GTRC) using a GT table, and a second translation from a GTRC to a PC using a GTRC table. Each transfer node connecting the originating node with the destination node performs a similar global title number translation to identify the next step. Each node knows only to forward a received signal with a particular global title number toward a certain network or direction. Of course, at some point on the journey of the signal, a correct destination PC has to be provided so that the signal can reach its final destination. Thus, all of the STPs within a given network must include up-to-date information (i.e. GT and GTRC tables) correlating global title numbers with specific PC information. In some implementations, the GT and GTRC tables may be merged into a single “routing” table.
The standardisation organisation Internet Engineering Task Force (IETF) has formed a working group known as Signal Transport (SIGTRAN) to formulate and implement the specifications necessary for transporting SS7 signalling over an IP network. SIGTRAN based signalling networks are expected to be introduced commercially over the next few years. IP based signalling has a number of advantages including increased network capacity and reduced infrastructure and maintenance costs.
The structure of a SIGTRAN based signalling network resembles on the logical level a full mesh (although the physical network may be of another structure, e.g. an SDH ring). This structure is illustrated in FIG. 2. There is no equivalent of the SS7 STP node and instead every SEP communicates on the logical level directly with every other SEP. The reason STPs are not needed is because routing is done at the IP level, thus allowing the physical and the logical networks to be different. (Some vendors may choose to include IP/STP nodes in SIGTRAN network, the STP nodes performing routing based upon SS7 addresses.)
One drawback of the SIGTRAN network, especially if the network consists of many nodes, is the fact that every SEP needs to have an SCCP GT/GTRC routing table(s) and a further IP address table mapping destination PCs to IP addresses (although again these tables may be merged into a single table or demerged into additional linked tables). Thus, a SEP needs to know the IP address (or addresses as an STP may “posses” multiple IP addresses to provide a degree of redundancy) of nearly every other SEP in the network. Maintaining these address tables, especially in situations when the network is growing rapidly (as will be the case in the years immediately following the first introduction of SIGTRAN), will be very expensive for the operator. Changing the IP address(es) of a SEP manually would also be difficult or even impossible in practice due to the major reconfiguration of the rest of the signalling network that this would require.
Even with today's SS7 networks, operators would like to reduce operating costs which are high due to the need to maintain the large global title and PC routing tables. If network management costs were to increase further with the introduction of SIGTRAN networks, many operators may not find SIGTRAN to be a feasible alternative.
The following are concerned with distributing routing information in the data plane: WO 200207331; The ‘Hello protocol’ described in the OSPF protocol (e.g. RFC 1247 July 1991); The ATM Forum P-NNI spec, para 5.6.